Quake: All the sources used above agree that quake stems from O.E. cwacian / cweccan.

Setting aside the uncertainty between Old English cwavien and Low German quabbeln being the source, it appears to be true that quaver and quake are related, with, according to ODO, the latter possibly being the source of the former. However, there's no mention of waver anywhere besides dictionary.reference.com.

Google, on the other hand, points to a couple of sources that lend credence to the portmanteau idea.

The word-formation process du jour in American English is blending, that is, combining two existing words to make a new word. A couple of blends formed in the Middle English period (1150 to 1500) have survived into Modern English, e.g., scrawl (sprout + crawl) and quaver (quake + waver), as well as a couple from Early Modern English (1500 to 1800), dumbfound (dumb + confound) and apathetic (apathy + pathetic).

However, among the numerous probable blends from ME are scroll from escrow + roll; scrawl from sprout + crawl; and quaver from quake + waver.

To conclude, while the (online) dictionaries do not mention the possibility of quaver being a blended word, there appear to be a few credible sources that do. There are, however, no authoritative sources that support this claim.